Browsing by Author "Simkins, Lauren M."
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Item Characteristics of the deforming bed: till properties on the deglaciated Antarctic continental shelf(Cambridge University Press, 2018) Halberstadt, Anna Ruth W.; Simkins, Lauren M.; Anderson, John B.; Prothro, Lindsay O.; Bart, Philip J.Contemporary ice stream flow is directly linked to conditions at the ice/bed interface, yet this environment is logistically difficult to access. Instead, we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated Antarctic continental shelf. We test currently-accepted hypotheses surrounding subglacial processes and till properties with a Ross Sea dataset. Till shear strengths indicate a continuum of simultaneous processes acting at the bed, rather than discrete ‘deformation’ and ‘lodgement’ end-members. We identify a threshold water content representing saturated pore spaces, leading to basal sliding and meltwater channelization. Based on observations of till properties relative to glacial landforms, we challenge the assumption that low shear strength is linked to intense deformation. Spatial variability in landform morphology reflects variability in deforming processes at the sub-ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line. Regional till properties generally correlate with seafloor geology and deglacial history; the western Ross Sea is characterized by higher and more variable shear strengths and water contents, while lower-shear strength till was preserved in the Eastern Basin. These observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves.Item Diagnosing ice sheet grounding line stability from landform morphology(Copernicus Publications, 2018) Simkins, Lauren M.; Greenwood, Sarah L.; Anderson, John B.The resilience of a marine-based ice sheet is strongly governed by the stability of its grounding lines, which are in turn sensitive to ocean-induced melting, calving, and flotation of the ice margin. Since the grounding line is also a sedimentary environment, the constructional landforms that are built here may reflect elements of the processes governing this dynamic and potentially vulnerable environment. Here we analyse a large dataset (n=6275) of grounding line landforms mapped on the western Ross Sea continental shelf from high-resolution geophysical data. The population is divided into two distinct morphotypes by their morphological properties: recessional moraines (consistently narrow, closely spaced, low amplitude, symmetric, and straight) and grounding zone wedges (broad, widely spaced, higher amplitude, asymmetric, sinuous, and highly variable). Landform morphotypes cluster with alike forms that transition abruptly between morphotypes both spatially and within a retreat sequence. Their form and distribution are largely independent of water depth, bed slope, and position relative to glacial troughs. Similarly, we find no conclusive evidence for morphology being determined by the presence or absence of an ice shelf. Instead, grounding zone wedge construction is favoured by a higher sediment flux and a longer-held grounding position. We propose two endmember modes of grounding line retreat: (1) an irregular mode, characterised by grounding zone wedges with longer standstills and accompanied by larger-magnitude retreat events, and (2) a steady mode, characterised by moraine sequences that instead represent more frequent but smaller-magnitude retreat events. We suggest that while sediment accumulation and progradation may prolong the stability of a grounding line position, progressive development of sinuosity in the grounding line due to spatially variable sediment delivery likely destabilises the grounding position by enhanced ablation, triggering large-magnitude retreat events. Here, the concept of “stability” is multifaceted and paradoxical, and neither mode can be characterised as marking fast or slow retreat. Diagnosing grounding line stability based on landform products should be considered for a wider geographic range, yet this large dataset of landforms prompts the need to better understand the sensitivity of marine-based grounding lines to processes and feedbacks governing retreat and what stability means in the context of future grounding line behaviour.Item Holocene reconfiguration and readvance of the East Antarctic Ice Sheet(Springer Nature, 2018) Greenwood, Sarah L.; Simkins, Lauren M.; Halberstadt, Anna Ruth W.; Prothro, Lindsay O.; Anderson, John B.How ice sheets respond to changes in their grounding line is important in understanding ice sheet vulnerability to climate and ocean changes. The interplay between regional grounding line change and potentially diverse ice flow behaviour of contributing catchments is relevant to an ice sheet’s stability and resilience to change. At the last glacial maximum, marine-based ice streams in the western Ross Sea were fed by numerous catchments draining the East Antarctic Ice Sheet. Here we present geomorphological and acoustic stratigraphic evidence of ice sheet reorganisation in the South Victoria Land (SVL) sector of the western Ross Sea. The opening of a grounding line embayment unzipped ice sheet sub-sectors, enabled an ice flow direction change and triggered enhanced flow from SVL outlet glaciers. These relatively small catchments behaved independently of regional grounding line retreat, instead driving an ice sheet readvance that delivered a significant volume of ice to the ocean and was sustained for centuries.Item Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica(Copernicus Publications, 2016) Halberstadt, Anna Ruth W.; Simkins, Lauren M.; Greenwood, Sarah L.; Anderson, John B.Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.Item Rapid retreat of Thwaites Glacier in the pre-satellite era(Springer Nature, 2022) Graham, Alastair G.C.; Wåhlin, Anna; Hogan, Kelly A.; Nitsche, Frank O.; Heywood, Karen J.; Totten, Rebecca L.; Smith, James A.; Hillenbrand, Claus-Dieter; Simkins, Lauren M.; Anderson, John B.; Wellner, Julia S.; Larter, Robert D.Understanding the recent history of Thwaites Glacier, and the processes controlling its ongoing retreat, is key to projecting Antarctic contributions to future sea-level rise. Of particular concern is how the glacier grounding zone might evolve over coming decades where it is stabilized by sea-floor bathymetric highs. Here we use geophysical data from an autonomous underwater vehicle deployed at the Thwaites Glacier ice front, to document the ocean-floor imprint of past retreat from a sea-bed promontory. We show patterns of back-stepping sedimentary ridges formed daily by a mechanism of tidal lifting and settling at the grounding line at a time when Thwaites Glacier was more advanced than it is today. Over a duration of 5.5 months, Thwaites grounding zone retreated at a rate of >2.1 km per year—twice the rate observed by satellite at the fastest retreating part of the grounding zone between 2011 and 2019. Our results suggest that sustained pulses of rapid retreat have occurred at Thwaites Glacier in the past two centuries. Similar rapid retreat pulses are likely to occur in the near future when the grounding zone migrates back off stabilizing high points on the sea floor.